1. Field of the Invention
The present invention relates to an image processing apparatus suitable for an electronic file, facsimile, reader or digital copier.
2. Description of the Prior Art
In a prior art apparatus which uses a solid-state image pick-up device such as a CCD to read pixel density of an image to reproduce the image, a function of background elimination in which background color of a document (for example, a newspaper) is not reproduced but only characters are sharply reproduced may be equipped to impart intelligency.
The methods for doing so are classified as follows:
1 The background color is sequentially detected and a binarization threshold is set to be higher than the background density.
2 An entire area of the document is read in advance, the background color level is detected from a density histogram and the binarization threshold is set accordingly. In the method 1, it is difficult to follow an abrupt change in the image density and a portion of character information is dropped or the background color is partially reproduced. Thus, it allows real-time processing but accuracy is low.
On the other hand, the latter or pre-scan method 2 allows high accuracy processing because it can correctly detect the background color. However, since the document has to be read twice, a double length of time is required and this method is not suitable to a high speed apparatus. In addition, the apparatus is complex because it must prepare the density histogram of the entire document.